The present invention relates to the offset printing blankets, and more particularly, to tubular offset lithographic printing blankets and methods for manufacturing the same.
A web offset printing press typically includes a plate cylinder, a blanket cylinder and an impression cylinder supported for rotation in the press. The plate cylinder carries a printing plate having a rigid surface defining an image to be printed. The blanket cylinder typically carries a printing blanket having an outer print layer, for example of rubber, which contacts the printing plate at a nip between the plate cylinder and the blanket cylinder. A web to be printed moves through a nip between the blanket cylinder and the impression cylinder. Ink is applied to the surface of the printing plate on the plate cylinder. An inked image is picked up by the printing blanket at the nip between the blanket cylinder and the plate cylinder, and is transferred from the printing blanket to the web at the nip between the blanket cylinder and the impression cylinder. The impression cylinder can be another blanket cylinder for printing on the opposite side of the web.
A conventional printing blanket is manufactured as a flexible flat sheet. Such a printing blanket is mounted on a blanket cylinder by wrapping the sheet around the blanket cylinder and by attaching the opposite ends of the sheet to the blanket cylinder in an axially extending gap in the blanket cylinder. The adjoining opposite ends of the sheet define a gap extending axially along the length of the printing blanket. The gap moves through the nip between the blanket cylinder and the plate cylinder, and also moves through the nip between the blanket cylinder and the impression cylinder, each time the blanket cylinder rotates.
When the leading and trailing edges of the gap at the printing blanket move through the nip between the blanket cylinder and an adjacent cylinder, pressure between the blanket cylinder and the adjacent cylinder is relieved and established, respectively. The repeated relieving and establishing of pressure at the gap causes vibrations and shock loads in the cylinders and throughout the printing press. Such vibrations and shock loads detrimentally affect print quality. For example, at the time that the gap relieves and establishes pressure at the nip between the blanket cylinder and the plate cylinder, printing may be taking place on the web moving through the nip between the blanket cylinder and the impression cylinder. Any movement of the blanket cylinder or the printing blanket caused by the relieving and establishing of pressure at that time can smear the image which is transferred from the printing blanket to the web. Likewise, when the gap in the printing blanket moves through the nip between the blanket cylinder and the impression cylinder, an image being picked up from the printing plate by the printing blanket at the other nip can be smeared. The result of the vibrations and shock loads caused by the gap in the printing blanket has been an undesirably low limit to the speed at which printing presses can be run with acceptable print quality.
In response to these deficiencies in conventional flat printing blankets, gapless tubular printing blankets were developed by the assignee of the present invention. These gapless tubular printing blankets are described, for example, in U.S. Pat. Nos. 5,768,990, 5,553,541, 5,440,981, 5,429,048, 5,323,702, and 5,304,267. These tubular blankets however have required that the print layer and compressible layers be supported by a stiff inner sleeve, for example made of nickel. The tubular blankets thus were not flexible, in that the inner surface of the sleeve could not contact itself or collapse without damaging the layers of the blanket. The tubular blankets thus need to be stored in a tubular shape, taking up valuable space in a press room or print shop.
U.S. Pat. No. 5,654,100 discloses an offset rubber-blanket sleeve with rubber as a base material, reinforced by layer inserts embedded therein, such as a spiral winding to provide similar strength to a fiberglass or metallic sleeve. The sleeve is thus not collapsible.
Commonly-assigned U.S. Pat. No. 6,257,140, which is hereby incorporated by reference herein, describes gapless tubular printing blankets produced continuously and cut to length as desired. The sleeve and print layer are xe2x80x9ccontinuouslyxe2x80x9d formed in that the sleeve forming station continues to form an additional portion of the sleeve while the print layer forming station applies the print layer to the previously formed portion of the sleeve. Wound tapes or cross-head extruders are used to apply various layers.
Commonly-assigned U.S. Pat. No. 6,538,970 discloses a machine for winding a sleeve, and is also incorporated by reference herein.
Commonly-assigned U.S. patent application Ser. No. 09/716,696, which is hereby incorporated by reference herein, provides for ribbon casting of materials to form various layers of a tubular printing blanket. xe2x80x9cRibbon castingxe2x80x9d occurs when a liquid material is deposited from a stationary source onto a rotating and translating substrate or that a liquid is deposited from a rotating source onto a translating substrate. A continuous ribbon of liquid material thus can be placed on the substrate. Urethane is used in the ribbon casting process. The urethane sets after a certain time.
All of the sleeves in the above-mentioned patent applications were designed to be stiff, typically being made of metal.
The present invention provides a device for manufacturing a flexible continuous printing blanket comprising:
a base;
an application layer located directly on the base;
a polymer applicator applying a polymer layer over the application layer so as to define a flexible sleeve layer, at least one of the application layer and the polymer layer being an innermost layer of the sleeve layer; and
a print layer applicator applying a print layer over the flexible sleeve layer.
The flexible sleeve of the present invention permits for better storage of blankets.
Preferably, a compressible layer applicator is located between the polymer layer and print layer applicators. The compressible layer applicator preferably applies a radiation-curing polymer that is a compressible liquid polymer, such as urethane mixed with microspheres, carbon dioxide, a blowing agent or water, for example.
Preferably, the radiation-curing polymer is polyurethane, and the radiation source is ultraviolet light. An electron beam also may be used for curing the polymer.
The sleeve preferably is made of urethane, for example a self-cure or radiation-curing urethane. A polyurethane layer with a hardness of at least 70 Shore A and most preferably a hardness of about 70 Shore D is preferred as the sleeve material.
The flexible application layer may be part of the sleeve, and may be made of a pre-fabricated tape that wraps around the rotating base. The tape may be a polyurethane film with a hardness of at least 70 Shore A and most preferably a hardness of about 70 Shore D is preferred as the sleeve material.
Alternately, the flexible application layer may be a release layer separating the base from the polymer applied by the polymer applicator.
The release layer may be for example a TEFLON tape which is removed from the flexible sleeve layer.
The present device preferably includes a rotation device for rotating the base, and the base and rotation device may be similar to the base devices used to form blankets in incorporated-by-reference U.S. Pat. Nos. 6,257,140 and 6,538,970 and U.S. application Ser. No. 09/716,696. These devices as a plurality of slats which push the sleeve so as permit a continuous manufacture.
Optional surface finishers for smoothing the surface may be located after the various applicators.
The sleeve may be formed continuously, so that a cutting device may be provided to cut the sleeve when a desired sleeve length is reached.
The present invention also provides a method for forming a tubular printing blanket comprising the steps of:
applying an application layer to a base;
applying a polymer over the application layer so as to form as flexible tubular sleeve, at least one of the application layer and the polymer being an innermost layer of the tubular sleeve; and
applying a print layer over the flexible tubular sleeve.
The method preferably includes forming a compressible layer over the flexible tubular sleeve and under the print layer.
The method may include removing the flexible application layer.
The applying of the flexible application layer may include winding a tape around the rotating base.
The method preferably further includes rotating the base.
The compressible layer may be a radiation curable polymer of a compressible material, for example UV-curable urethane. A curing step then preferably takes place in a few seconds, although times up to 5 minutes are possible.
A smoothing step may be provided both after and before the curing step.
The flexible sleeve can be pre-manufactured, and then used in a separate process to make the blanket. Alternately, the blanket can be made in a single continuous process.
Preferably, the print layer, compressible layer and flexible sleeve are made of urethane, and a reinforcing layer is provided between the compressible layer and the print layer. The reinforcing layer is also preferably made of urethane.
The reinforcing layer preferably is made of a high durometer urethane of greater than 70 shore A, most preferably about 70 shore D.
The print layer preferably is made of a urethane with a durometer of less than 80 shore A and most preferably of about 60 shore A.
The method of the present invention preferably includes compressing the printing blanket so that two different circumferential points of an inner surface of the sleeve when round contact each other. This permits for example storage of the sleeve. Preferably, most of the inner surface contacts itself. Various fold shapes are possible, depending on the storage area available.
The method also includes sliding the printing blanket over a cantilevered cylinder of an offset lithographic printing press.
The present invention also provides an offset printing blanket comprising:
a flexible and collapsible inner sleeve made of polymeric material, the sleeve being the innermost layer of the blanket; and
a print layer disposed over the flexible inner sleeve.
Preferably, a compressible layer is disposed between the print layer and the inner sleeve, and a reinforcing layer is disposed over the compressible layer and under the print layer.
The flexible inner sleeve may be made of urethane.